Nullable backing field when explicitely annotated

[Rubidium37]

Nullable backing field when explicitely annotated

• Related to [Proposal]: field keyword in properties #8635

Summary

I'm actively using the field keyword in my projects.
I'm using it both with class types and values types.

I found out that a possible feature is missing: having the backing field nullable even when the property is not.

It somehow already works for class types, by adopting nullable reference types annotations.
It is not supported for value types.

It would be nice if the specification will include some practice (attribute annotation, explicit field type declaration, etc..) to specify the type of the backing field as nullable.

If a code generator is involved to generate the backing field, than it would be enough to append the ? symbol after the type when generating code, if (and only if) the MaybeNull attribute is annotated.

To make it clear, below you can find two code samples that depict the use case.

Code samples

1. Declare a readonly property whose type is a struct (e.g.: int).
2. Implement the get accessor with null-coalesing assignment on the field keyword.
3. Mark the property with [Field: MaybeNull]

Minimal code:

using System;
using System.Diagnostics.CodeAnalysis;
public class C
{
	[field: MaybeNull]
	public int TodayYear => field ??= DateTime.Today.Year;
}

Expected Behavior:

The code should compile fine.
I expect that the MaybeNull attribute and/or the coalesce assignment operator (??=) hint the compiler to make the backing field nullable; that is, Nullable<int>.
Thus, it should produce something equivalent to:

using System;
//using System.Diagnostics.CodeAnalysis;
public class C
{
	private int? backingfield_TodayYear
	public int TodayYear => backingfield_TodayYear ??= DateTime.Today.Year;
}

Actual Behavior:

I get compiler error CS0019: Operator '??=' cannot be applied to operands of type 'int' and 'int'

For comparison, you can try the code snippet on Sharplab IO
It show both the ideal code and the manually implemented equivalent.

[huoyaoyuan]

Nullable value type is a different type. It can't be affected by annotation attributes, which only affects nullable reference type. Declaring the backing field with a different type is out of scope currently.

Issues in csharplang repo are reserved for language design team. Please use discussions to express ideas.

[jnm2]

#133 would provide one solution to this, where you would write:

public class C
{
    public int TodayYear { int? field; get => field ??= DateTime.Today.Year; }
}